This invention relates to proportional gas flow sampling equipment, and involves equipment which is particularly adapted for use with or in the equipment disclosed and claimed in copending U.S. patent application Ser. No. 06/272,204 filed June 10, 1981, in the names of William H. Clingman, Jr. and Lyn R. Kennedy, now U.S. Pat. No. 4,396,299, assigned to the same assignee as this application and entitled Method and Apparatus For Determining Total Energy Flow In A Gas Line. While the present invention is useful in other applications as well, the equipment disclosed in said patent application forms a good illustration of a situation in which it is desired to take a proportionally constant flowing sample from a flowing gas stream in a gas line for purposes of making additional measurements of various sorts on it. Proportionality of the sample is important in order to avoid variable calibration factors, and the challenge is to provide equipment which takes a proportionally constant sample over an appreciable range of flow rates in the main gas line.
A baffle arrangement may be provided in a main gas flow line such that a constant proportion of the total flow enters a bypass line upon diversion by the baffle. It is possible to use just baffles for proportional flow sampling, at least over a narrow range of flow rates, and a sampler relying solely on baffles has a very simple piping arrangement. An advantage of the baffle system is that little pressure drop is introduced into the main line. Thus the baffles can be used with "6 ounce" gas in a distribution line. A disadvantage of the baffles is that the proportional cut should not be less than 10-20% of the total flow. If one tries to extract too small a fraction, then changes in the flow profile as a function of Reynolds number will have a significant effect and reduce accuracy.
It is also possible to construct and use a sampler relying solely upon an orifice arrangement without baffles. Such an arrangement is disclosed in the above mentioned patent application. An advantage of the orifice sampling system is that a very small cut can be taken from the main line, such as one part in one thousand.
A disadvantage of either a baffle system or an orifice plate system by themselves is that the dynamic range is limited in an energy flowmeter application. In the latter instrument a proportional flow sampler is combined with a modified titrator-type apparatus for determining energy content or calorific value of the kind generally disclosed in Clingman U.S. Pat. Nos. 3,777,562 issued Dec. 11, 1973; 4,062,236 issued Dec. 13, 1977; 4,125,018 issued Nov. 14, 1978; and, 4,125,123 also issued Nov. 14, 1978. In the energy meter section of the equipment the sample flow is mixed with air and burned. There will only be about a five to one range of flows that can be accomodated with a single burner design. Also, the greater the flow range at the burner the less the inherent accuracy will be.
By combining the baffles with the orifice plates system a two-stage proportional flow sampler is obtained that has both good dynamic range as well as good accuracy.